1. Field of the Invention
The present invention relates to a work apparatus such as a robot that performs work, and particularly relates to a calibration method for calibrating the optical-axis direction of a distance measurement unit that the work apparatus has, and an offset amount.
2. Description of the Related Art
In the field of industrial robots used for work such as assembly or processing, in a case in which operation is performed with respect to an object whose position is indefinite, it is necessary to measure the position of the object, which is a work target. Image information is often used for two-dimensional position detection, and for a case in which the height of the work target is also indefinite, there are various methods using three-dimensional measurement means.
In a case of causing a robot to work based on the combination of image information and distance measurement, it is necessary to know how the measurement result obtained by a distance measurement unit (distance sensor) corresponds to a robot coordinate system that the robot internally has. Then, for that purpose, it is necessary to obtain a relative placement positional relationship between the robot (work position thereof) and the distance sensor as calibration data.
Measures for addressing the above problem are proposed in the following Patent Documents 1 to 6.    Patent Document 1: Japanese Patent Laid-Open No. 11-123678    Patent Document 2: Japanese Patent Laid-Open No. 2005-271103    Patent Document 3: Japanese Patent No. 03239277    Patent Document 4: Japanese Patent Laid-Open No. 05-280927    Patent Document 5: Japanese Patent Laid-Open No. 10-103937    Patent Document 6: Japanese Patent No. 04021413
In Patent Document 1, the end of a welding torch of a welding robot is caused to touch a reference position on a workpiece in teaching, and a laser illumination position of a laser sensor for distance measurement is caused to match with the reference position in teaching, thereby obtaining the optical-axis direction of the distance sensor and an offset amount. Further, in Patent Document 2, calibration is performed by causing a robot to touch a plurality of points on a plane jig whose world coordinates are known in teaching, and establishing correspondence between the result of three-dimensional measurement and the robot coordinate system.
These Patent Documents 1 and 2 disclose methods for calibration by manually moving a robot in teaching and causing the robot to touch a reference position.
Patent Document 3 discloses that by placing the optical axis of a distance sensor so as to match the center axis of a robot hand, correspondence between the robot coordinate system and the distance sensor coordinate system is established, without calibration. This is a method for addressing the problem, through the position at which the sensor is placed.
Patent Document 4 discloses that a plate having a marker whose position is known is attached to a robot arm, laser slit light from a projector placed in a global position is emitted on the plate, an image thereof is taken with a camera, and coordinate calibration is performed. Since the world coordinate position of the marker is determined by a robot command, correspondence between the image coordinates of the laser slit and the robot coordinates are decided.
Further, in Patent Document 5, a tilted plate is rotated having a rotation axis at the same position as the design position of the distance sensor laser optical axis, and the angle of the optical axis relative to a designed value is calculated from the difference between measured values. This method requires a jig with which highly accurate operation and attachment accuracy are assured.
In Patent Document 6, the optical-axis direction of a camera mounted to an arm is obtained by moving the arm while maintaining a target in a working area so as to be in the image center using visual servoing. In this conventional example, only the optical-axis direction of the camera is obtained, and calculation for obtaining an offset amount relative to the robot hand is not referred to.
With a calibration method achieved by teaching a robot to touch as disclosed in Patent Documents 1 and 2, since physical contact of the robot is involved, there is always the risk of the robot being broken, and in a case of requiring calibration with accuracy, a worker needs to be skilled.
Further, with the apparatus configuration for causing the center axis of a robot hand and the distance sensor optical axis to match as disclosed in Patent Document 3, restrictions will be imposed when designing a holding mechanism and the like. Furthermore, since an attachment error is not allowed when the sensor is attached, high assembly accuracy is needed.
Further, in Patent Document 4, the projector of laser slit light is placed in a global position, and accordingly a measurable area is limited. Further, in order to make the marker position of the plate attached to the robot arm known in the robot coordinate system, the marker position on the plate needs to be decided with high accuracy, and the attachment accuracy of the plate is also required.
Further, although Patent Document 5 discloses a calibration method using a special jig, a jig that is created with high accuracy and operates with high accuracy is needed, and the jig also needs to be attached to an apparatus accurately. Such a jig is expensive, and furthermore, placement thereof is complicated.